The HTTP spec says it's 'Content-Type'

Fixes #67

lib/mock-ajax.js

@@ -193,7 +193,7 @@ WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
         this.responseText = response.responseText || "";
         this.readyState = 4;
         this.responseHeaders = response.responseHeaders ||
-          {"Content-type": response.contentType || "application/json" };
+          {"Content-Type": response.contentType || "application/json" };
 
         this.onload();
         this.onreadystatechange();

spec/javascripts/mock-ajax-toplevel-spec.js

@@ -18,7 +18,7 @@ describe("Jasmine Mock Ajax (for toplevel)", function() {
     onreadystatechange = function() {
       if (this.readyState == (this.DONE || 4)) { // IE 8 doesn't support DONE
         if (this.status == 200) {
-          if (this.responseHeaders['Content-type'] === 'application/json') {
+          if (this.responseHeaders['Content-Type'] === 'application/json') {
             this.response = JSON.parse(this.responseText);
           } else {
             this.response = this.responseText;
@@ -407,7 +407,7 @@ function sharedAjaxResponseBehaviorForZepto_Success(context) {
     });
 
     it("should have the expected content type", function() {
-      expect(xhr.getResponseHeader('Content-type')).toEqual(context.contentType);
+      expect(xhr.getResponseHeader('Content-Type')).toEqual(context.contentType);
     });
 
     it("should have the expected response text", function() {
@@ -432,7 +432,7 @@ function sharedAjaxResponseBehaviorForZepto_Failure(context) {
     });
 
     it("should have the expected content type", function() {
-      expect(xhr.getResponseHeader('Content-type')).toEqual(context.contentType);
+      expect(xhr.getResponseHeader('Content-Type')).toEqual(context.contentType);
     });
 
     it("should have the expected response text", function() {

spec/javascripts/webmock-style-spec.js

@@ -37,7 +37,7 @@ describe("Webmock style mocking", function() {
 
   it("should set the contentType", function() {
     sendRequest(fakeGlobal);
-    expect(response.responseHeaders['Content-type']).toEqual('application/json');
+    expect(response.responseHeaders['Content-Type']).toEqual('application/json');
   });
 
   it("should set the responseText", function() {