fetch-mock: Add type for matching headers.

types/fetch-mock/fetch-mock-tests.ts

@@ -1,6 +1,11 @@
 import * as fetchMock from "fetch-mock";
 
 fetchMock.mock("http://test.com", 200);
+fetchMock.mock("http://test.com", 200, {
+    headers: {
+        test: "header"
+    }
+});
 fetchMock.mock(/test\.com/, 200);
 fetchMock.mock(() => true, 200);
 fetchMock.mock((url, opts) => true, 200);

types/fetch-mock/index.d.ts

@@ -96,6 +96,10 @@ declare namespace fetchMock {
          * http method to match
          */
         method?: string;
+        /**
+         * key/value map of headers to match
+         */
+        headers?: { [key: string]: string };
         /**
          * as specified above
          */