fix(lib): Adds missing context block to CSR

When comparing a CSR generated with the same parameters using openssl,
I noticed that the CSR generated by rcgen was missing attributes data.

OpenSSL:
```
Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: C=US, ST=., L=., O=., OU=., CN=krustlet
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:3c:2b:64:c2:0e:8f:fa:86:b2:5c:f1:66:82:a7:
                    73:48:37:3b:d7:f0:12:25:0e:29:c2:65:20:1f:a5:
                    60:36:52:70:ba:60:7e:00:e0:92:74:fe:dd:0f:8a:
                    28:0a:6e:1b:41:46:10:96:72:54:5f:3c:d1:e5:86:
                    59:9b:61:a1:e3
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        Attributes:
            a0:00
    Signature Algorithm: ecdsa-with-SHA256
         30:45:02:20:50:a9:e6:11:9c:46:4b:c2:85:74:f9:58:30:26:
         4f:9c:d2:18:5f:33:fb:b6:74:44:3b:47:b1:2c:e8:cc:14:ac:
         02:21:00:eb:92:eb:b8:98:89:2e:d2:0e:fe:9c:af:c7:fe:60:
         e5:56:af:92:06:37:0f:71:38:1d:19:ba:04:88:3d:b8:b3
```

rcgen:
```
Certificate Request:
    Data:
        Version: 0 (0x0)
        Subject: C=US, ST=., L=., O=., OU=., CN=krustlet
        Subject Public Key Info:
            Public Key Algorithm: id-ecPublicKey
                Public-Key: (256 bit)
                pub:
                    04:02:79:50:53:29:21:16:25:a2:bf:f7:24:41:0d:
                    12:7b:bf:60:96:b2:b7:fc:c0:2a:8a:16:69:bc:7e:
                    2d:95:b1:c8:75:23:07:d5:35:89:14:be:56:d3:6b:
                    2f:b1:e5:1a:39:be:74:bb:25:c8:f8:4a:6f:cf:9a:
                    27:39:2c:96:66
                ASN1 OID: prime256v1
                NIST CURVE: P-256
        Attributes:
    Signature Algorithm: ecdsa-with-SHA256
         30:45:02:20:7e:96:16:a3:b3:8b:21:25:ce:a7:8a:e0:53:19:
         cb:3e:24:02:25:73:6a:c8:e7:a0:15:3f:96:c4:b6:bc:50:24:
         02:21:00:b6:0e:d8:5d:1c:76:32:78:b7:0a:da:24:c3:58:3a:
         4d:9c:8e:16:e8:5e:d3:13:61:ec:e0:2c:77:48:b7:d1:23
```

This turned out to be missing the following section when looking at
`openssl asn1parse`:

```
183:d=2  hl=2 l=   0 cons:   cont [ 0 ]
```

To fix this, I changed the writing of the CSR to always write the context
tag, even if no extended attributes were given.

NOTE: I found this while trying to use the generated CSR with Kubernetes, and
apparently the Go library didn't like it. However, upon further investigation,
empty attributes still expect an empty context to be set, so this appears to
be compliant with the spec

src/lib.rs

@@ -573,21 +573,24 @@ impl Certificate {
 			});
 			// Write subjectPublicKeyInfo
 			self.key_pair.serialize_public_key_der(writer.next());
-			// Write extensions
-			if !self.params.subject_alt_names.is_empty() {
-				writer.next().write_tagged(Tag::context(0), |writer| {
-					writer.write_sequence(|writer| {
-						let oid = ObjectIdentifier::from_slice(OID_PKCS_9_AT_EXTENSION_REQUEST);
-						writer.next().write_oid(&oid);
-						writer.next().write_set(|writer| {
-							writer.next().write_sequence(|writer| {
-								// Write subject_alt_names
-								self.write_subject_alt_names(writer.next());
-							});
-						});
-					});
-				});
-			}
+            // Write extensions
+            // To match what openssl outputs and what some asn1 decoders expect, always write the
+            // context tag even if things are empty
+            writer.next().write_tagged(Tag::context(0), |writer| {
+                if !self.params.subject_alt_names.is_empty() {
+                    writer.write_sequence(|writer| {
+                        let oid = ObjectIdentifier::from_slice(OID_PKCS_9_AT_EXTENSION_REQUEST);
+                        writer.next().write_oid(&oid);
+                        writer.next().write_set(|writer| {
+                            writer.next().write_sequence(|writer| {
+                                // Write subject_alt_names
+                                self.write_subject_alt_names(writer.next());
+                            });
+                        });
+                    });
+                }
+            });
+
 		});
 	}
 	fn write_cert(&self, writer :DERWriter, ca :&Certificate) -> Result<(), RcgenError> {