response Type is set to wrong one(json) when the actual response is a string with Content-Type "text/plain"

**I'm submitting a bug report**
- **Library Version:**
0.11.0

**Please tell us about your environment:**
- **Operating System:**
Windows [7] professional
- **Node Version:**
4.6.1


- **NPM Version:**
3.8.9


- **JSPM OR Webpack AND Version**
JSPM 0.16.32


- **Browser:**
IE 11
- **Language:**
ESNext(ES - 6), angularjs

**Current behavior:**
In IE-11 the headers.headers['Content-Type'] of HttpResponseMessage is always being undefined. So the response type is set to wrong one(json), Even if I am getting the response from the server as a string with content-type text/plain. So getting "invalid character" Error when tried to parse the string response#151

```
if (this.responseType === 'json') {
this._content = JSON.parse(this.response, this.reviver);
return this._content;
}
```

**Expected/desired behavior:**

- **What is the expected behavior?**
It should not parse the response when the response is a string with content-type text/plain. So as to prevent the "Invalid character" error in IE-11.
- **What is the motivation / use case for changing the behavior?**

Start by tracing the responseType selection and the shown JSON.parse path in the HttpResponseMessage response handling, focusing on how IE 11 exposes the Content-Type header. Reproduce a text/plain response containing a string and verify that it is returned without JSON parsing, while JSON responses retain their existing behavior.

Written by the indexing model from the issue text.