Breaking News: responseTransformer becomes more powerful!

Breaking changes around responseTransformer!

In this PR #165, we realized that the responseTransformer feature added last release wasn't broad enough, responseTransformers now receive the raw response stream instead of just the json()-promise.

Code which relies on this feature will break, however the fix should be very simple:

Either the responseTransformer function is made `async` and to `await response.json()`, *or* if this syntax is not available, the existing code needs to be wrapped in `response.json().then(data => {/* existing implementation */})`.

Other Changes

• Remove restriction that only allows request bodies to be built for Mutation operations. #154 & #173
• Fix code sandbox examples #177
• Bug-fix: default to empty headers instead of undefined for IE #178
• Various docs typo fixes