fix: preserve explicit XHR Content-Type header (#250)

Fixes #33

Author: fatso83

lib/fake-xhr/index.js

@@ -910,11 +910,8 @@ function fakeXMLHttpRequestFor(globalScope) {
                     "Content-Type",
                 );
                 var FormDataConstructor = getFormDataConstructor();
-                if (this.requestHeaders[contentType]) {
-                    var value = this.requestHeaders[contentType].split(";");
-                    this.requestHeaders[contentType] =
-                        `${value[0]};charset=utf-8`;
-                } else if (
+                if (
+                    !this.requestHeaders[contentType] &&
                     !(
                         FormDataConstructor &&
                         data instanceof FormDataConstructor

lib/fake-xhr/index.test.js

@@ -677,26 +677,26 @@ describe("FakeXMLHttpRequest", function () {
             );
         });
 
-        it("does not override mime", function () {
+        it("preserves caller-provided Content-Type", function () {
             this.xhr.open("POST", "/");
             this.xhr.setRequestHeader("Content-Type", "text/html");
             this.xhr.send("Data");
 
-            assert.equals(
-                this.xhr.requestHeaders["Content-Type"],
-                "text/html;charset=utf-8",
-            );
+            assert.equals(this.xhr.requestHeaders["Content-Type"], "text/html");
         });
 
-        it("does not add new 'Content-Type' header if 'content-type' already exists", function () {
+        it("preserves caller-provided content-type casing and value", function () {
             this.xhr.open("POST", "/");
-            this.xhr.setRequestHeader("content-type", "application/json");
+            this.xhr.setRequestHeader(
+                "content-type",
+                "application/json; charset=iso-8859-1",
+            );
             this.xhr.send("Data");
 
             assert.isUndefined(this.xhr.requestHeaders["Content-Type"]);
             assert.equals(
                 this.xhr.requestHeaders["content-type"],
-                "application/json;charset=utf-8",
+                "application/json; charset=iso-8859-1",
             );
         });