Show Debug Toolbar Only For HTML Responses

[SmolSoftBoi]

Only show the debug toolbar for responses containing HTML, allowing other responses (such as JSON and XML) to show clean.

[lonnieezell]

I'll have to consider this more tonight, but one thing that's been on my mental to-do list is to make the toolbar capable of supporting multiple output formats, especially now that we have formatters in the API namespace. Will probably have to switch some things around to make that happen, though.

[lonnieezell]

Will merge this for now, since I'm not sure when the refactor will be able to happen.

[tianhe1986]

@EpicKris @lonnieezell
After this PR, I tried to use debug toolbar with default Home controller, but it didn't show.

I added var_dump($format); in DebugToolbar::after and found it was empty string.

So, may it be better to add default content-type header, at least for view() ?

PS: I use Windows 10 + Apache 2.4.23 + PHP 7.0.10.

[SmolSoftBoi]

@lonnieezell Should I update this pull request to account for an empty $format or should there be a default content-type header?

[lonnieezell]

@EpicKris There should be a default content-type header, and I could have sworn it already did that, but I haven't had a chance to look into it.