SpringMvcContract: Ignore RequestMapping headers with no value.

According to:

https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/web/bind/annotation/RequestMapping.html#headers--

the `headers` field can take several formats:

1. Require the header with a concrete value: `"My-Header=myValue"`
2. Require the header with any value: `"My-Header"`
3. Require the header is absent: `"!My-Header"`
4. Require the header with a negated value : `"My-Header!=myValue"`

This fixes `parseHeaders` for cases 2-4 (which previously threw a `StringIndexOutOfBoundsException`).

Fixes gh-1778.

spring-cloud-netflix-core/src/main/java/org/springframework/cloud/netflix/feign/support/SpringMvcContract.java

@@ -275,11 +275,13 @@ public class SpringMvcContract extends Contract.BaseContract
 		if (annotation.headers() != null && annotation.headers().length > 0) {
 			for (String header : annotation.headers()) {
 				int index = header.indexOf('=');
+				if (!header.contains("!=") && index >= 0) {
 					md.template().header(resolve(header.substring(0, index)),
 						resolve(header.substring(index + 1).trim()));
 				}
 			}
 		}
+	}
 
 	private Map<Class<? extends Annotation>, AnnotatedParameterProcessor> toAnnotatedArgumentProcessorMap(
 			List<AnnotatedParameterProcessor> processors) {

spring-cloud-netflix-core/src/test/java/org/springframework/cloud/netflix/feign/support/SpringMvcContractTests.java

@@ -332,6 +332,18 @@ public class SpringMvcContractTests {
 		assertEquals("bar", data.template().headers().get("X-Foo").iterator().next());
 	}
 
+	@Test
+	public void testProcessHeadersWithoutValues() throws Exception {
+		Method method = TestTemplate_HeadersWithoutValues.class.getDeclaredMethod("getTest",
+				String.class);
+		MethodMetadata data = this.contract
+				.parseAndValidateMetadata(method.getDeclaringClass(), method);
+
+		assertEquals("/test/{id}", data.template().url());
+		assertEquals("GET", data.template().method());
+		assertEquals(true, data.template().headers().isEmpty());
+	}
+
 	@Test
 	public void testProcessAnnotations_Fallback() throws Exception {
 		Method method = TestTemplate_Advanced.class.getDeclaredMethod("getTestFallback",
@@ -462,6 +474,11 @@ public class SpringMvcContractTests {
 		ResponseEntity<TestObject> getTest(@PathVariable("id") String id);
 	}
 
+	public interface TestTemplate_HeadersWithoutValues {
+		@RequestMapping(value = "/test/{id}", method = RequestMethod.GET, headers = { "X-Foo", "!X-Bar", "X-Baz!=fooBar" })
+		ResponseEntity<TestObject> getTest(@PathVariable("id") String id);
+	}
+
 	public interface TestTemplate_ListParams {
 		@RequestMapping(value = "/test", method = RequestMethod.GET)
 		ResponseEntity<TestObject> getTest(@RequestParam("id") List<String> id);