fix($templateRequest): ignore JSON Content-Type header and content

Normally, if there is a Content-Type header with the string "application/json", or else the content
looks sort of JSON-y, $http will attempt to deserialize the JSON into an object. $templateRequest
is intended to request markup, and as such should never attempt to parse JSON, regardless of the
headers or shape of the content.

Closes #5756
Closes #9619