Added messagingContentType

without this change you can by mistake use the http headers in a messaging headers which will fail to work
with this change we're adding the messagingContentType() method that allows to properly set the content type

fixes #296

spring-cloud-contract-spec/src/main/groovy/org/springframework/cloud/contract/spec/internal/Headers.groovy

@@ -34,6 +34,7 @@ class Headers {
 
 	@Delegate MediaTypes mediaTypes = new MediaTypes()
 	@Delegate HttpHeaders httpHeaders = new HttpHeaders()
+	@Delegate MessagingHeaders messagingHeaders = new MessagingHeaders()
 
 	Set<Header> entries = []
 
@@ -60,6 +61,10 @@ class Headers {
 		header(httpHeaders.contentType(), matching(contentType))
 	}
 
+	void messagingContentType(String contentType) {
+		header(messagingHeaders.messagingContentType(), matching(contentType))
+	}
+
 	/**
 	 * If for the consumer / producer you want to match exactly only
 	 * the root of content type. I.e. {@code application/json;charset=UTF8}

spring-cloud-contract-spec/src/main/groovy/org/springframework/cloud/contract/spec/internal/MessagingHeaders.groovy

@@ -0,0 +1,25 @@
+package org.springframework.cloud.contract.spec.internal
+
+import groovy.transform.CompileStatic
+import groovy.transform.EqualsAndHashCode
+import groovy.transform.ToString
+
+/**
+ * Contains most commonly used messaging headers
+ *
+ * @author Marcin Grzejszczak
+ * @since 1.1.2
+ */
+@CompileStatic
+@EqualsAndHashCode
+@ToString(includePackage = false)
+class MessagingHeaders {
+
+	/**
+	 * The Content Type of a message
+	 * @return
+	 */
+	String messagingContentType() {
+		return "contentType"
+	}
+}

spring-cloud-contract-verifier/src/test/groovy/org/springframework/cloud/contract/verifier/builder/MessagingMethodBodyBuilderSpec.groovy

@@ -40,6 +40,7 @@ def contractDsl = Contract.make {
 		body('''{ "bookName" : "foo" }''')
 		headers {
 			header('BOOK-NAME', 'foo')
+			messagingContentType(applicationJson())
 		}
 	}
 }
@@ -60,6 +61,7 @@ def contractDsl = Contract.make {
   ContractVerifierMessage response = contractVerifierMessaging.receive('activemq:output')
   assert response != null
   response.getHeader('BOOK-NAME')?.toString()  == 'foo'
+  response.getHeader('contentType')?.toString()  == 'application/json'
  and:
   DocumentContext parsedJson = JsonPath.parse(contractVerifierObjectMapper.writeValueAsString(response.payload))
   assertThatJson(parsedJson).field("bookName").isEqualTo("foo")
@@ -81,6 +83,7 @@ def contractDsl = Contract.make {
 					body('''{ "bookName" : "foo" }''')
 					headers {
 						header('BOOK-NAME', 'foo')
+						messagingContentType(applicationJson())
 					}
 				}
 			}
@@ -101,6 +104,8 @@ def contractDsl = Contract.make {
   assertThat(response).isNotNull();
   assertThat(response.getHeader("BOOK-NAME")).isNotNull();
   assertThat(response.getHeader("BOOK-NAME").toString()).isEqualTo("foo");
+  assertThat(response.getHeader("contentType")).isNotNull();
+  assertThat(response.getHeader("contentType").toString()).isEqualTo("application/json");
  // and:
   DocumentContext parsedJson = JsonPath.parse(contractVerifierObjectMapper.writeValueAsString(response.getPayload()));
   assertThatJson(parsedJson).field("bookName").isEqualTo("foo");