feat(http): add content-type override support for http request (#10211)

modules/@angular/http/src/static_request.ts

@@ -90,20 +90,41 @@ export class Request extends Body {
       }
     }
     this._body = requestOptions.body;
-    this.contentType = this.detectContentType();
     this.method = normalizeMethodName(requestOptions.method);
     // TODO(jeffbcross): implement behavior
     // Defaults to 'omit', consistent with browser
     // TODO(jeffbcross): implement behavior
     this.headers = new Headers(requestOptions.headers);
+    this.contentType = this.detectContentType();
     this.withCredentials = requestOptions.withCredentials;
     this.responseType = requestOptions.responseType;
   }
 
+  /**
+   * Returns the content type enum based on header options.
+   */
+  detectContentType(): ContentType {
+    switch (this.headers.get('content-type')) {
+      case 'application/json':
+        return ContentType.JSON;
+      case 'application/x-www-form-urlencoded':
+        return ContentType.FORM;
+      case 'multipart/form-data':
+        return ContentType.FORM_DATA;
+      case 'text/plain':
+      case 'text/html':
+        return ContentType.TEXT;
+      case 'application/octet-stream':
+        return ContentType.BLOB;
+      default:
+        return this.detectContentTypeFromBody();
+    }
+  }
+
   /**
    * Returns the content type of request's body based on its type.
    */
-  detectContentType() {
+  detectContentTypeFromBody(): ContentType {
     if (this._body == null) {
       return ContentType.NONE;
     } else if (this._body instanceof URLSearchParams) {