Fix callback execution when request fails

[dafortune]

Call end callback when there is an error

Fixes #5

[jpodwys]

Merged and published in NPM release 1.0.3. Thanks for the input!

[fraczak]

I experience the problem (in a browser via browserify) on version 1.7.2 :(

[jpodwys]

Superagent made some internally breaking changes in 1.7.0 and later. Please see #32 for more information on what's being done about it. If you don't need anything in 1.7.0 or later, please move to 1.6.1 until a resolution is reached. If you still encounter the error while using 1.6.1, please let me know.

I referenced this page from the linked issue.

[jpodwys]

@fraczak Can you please provide more information? Were you on an older version of superagent and everything was working well until you upgraded, or did you just come across this the first time you tried to use superagent-cache?

[jpodwys]

@fraczak If you can, I'd appreciate a code demo. I'm unable to reproduce this error, even on 1.7.2. Thanks!