When possible, set the Accept header appropriately

[candrews]

When the transport supports doing so, set the "Accept" request header to an appropriate value. Doing so informs the server what type of response the client is expecting.

Closes https://github.com/sockjs/sockjs-client/issues/540

[candrews]

The failing test is because the test doesn't have access to Travis secure variables: https://github.com/sockjs/sockjs-client/blob/ddae5a3eb1f482e2628977d3473ec4f3eea99bf2/build.sh#L7

It would pass if run by the main sockjs-client repo.

[candrews]

@brycekahle can you please take a look at this PR?

[brycekahle]

@candrews Apologies. I'll take a look at this soon. I just need to verify the Accept values are valid in all cases.