Change middleware to support rails app in subfolders

[thomet]

If two applications run on the same domain

first_rails_app -> in http:://any_domain/
second_rails_app -> in http://any_domain/seccond_application

and in the second one a error is occured then "__better_errors" from the second app should be called.

To provide this the regular expression for the better_errors_call-Method should listen on any "__better_errors" string on any page.

[ghost]

It's a super simple fix, but it solves the problem flawlessly. Moreover, test spec is included.
@thomet: thanks for your attention to the problem!

[qqshfox]

It works. Thanks for @thomet.

[grncdr]

I can confirm that this fixes #93 for me

[ur5us]

👍

[SeriousM]

I had a problem with rails_admin mounted under /admin.
better_errors was failing requesting http://localhost:3000/admin/__better_errors/123456/variables/...
therefor there was no error information about.
with this pull request the error is visible again!

thanks for that 👍

[ramigg]

👍
This pull request fixed the same issue I had with mountable engines I use.

Please merge it

[ghost]

@charliesome, any updates on this pull request?
There're at least 9 people waiting this fix.
Engines are the official way of some sort of DRY and wouldn't be removed in rails 4.
In addition to that, there're some popular gems which related on engines (for example, active_admin).
Could you comment on the weak points which you see of this solution so someone could improve it to merge to upstream?

[deiga]

👍