Make use of responseType=blob conditional

[vdonich]

At the moment, the polyfill sets responseType='blob' unconditionally.
However, for the purpose of using <link rel="preload" for non-blob responses, the above responseType shouldn't be set in some cases.

I'm not sure which way is the best, maybe adding a responseType to supports object and then exposing it for modification on fetch similarly to polyfill = true would make sense?

[dgraham]

Related to, and possible duplicate of, #481.

[JakeChampion]

Since #752 was merged, we no longer unconditionally set the responseType to blob.

I'll close this issue as I believe it is now resolved. If it is still an issue please comment and I will reopen and investigate :-)