Fix oauth2-redirect outside of the base swagger path

[alvarogf97]

Problem

When you use Oaut2 implicit flow, swagger ui sends an url that not includes the path where the swagger ui is located to the authorization page

If you include swaggo in your project like this:

router.GET("/swagger/*any", ginSwagger.WrapHandler(swaggerFiles.Handler))

You will get this redirect url in the oauth2 implicit flow:

{protocol}//{host}/oauth2-redirect.html

This url is not found due to his location is {protocol}//{host}/swagger/oauth2-redirect.html
actually

Relation issue
The issue is #115

Additional context
I add the minimun changes to make it work.

PD: sorry for the other changes fmt did his work :D and I think it could be worth

cc: @ubogdan @DeyiXu

The following images belong to my own project that can be a proof of how it works

Sorry for my broken english, I'm from Spain :D

[codecov]

Codecov Report

Merging #177 (c040683) into master (7da8e84) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #177   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           70        81   +11     
=========================================
+ Hits            70        81   +11     

Impacted Files	Coverage Δ
swagger.go	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7da8e84...c040683. Read the comment docs.

[ubogdan]

LGTM

[ubogdan]

@alvarogf97 Thanks for your contribution.

[alvarogf97]

thanks for merging @ubogdan

Could you create a new package release? Thanks!

[ubogdan]

added v1.3.3

[DeyiXu]

Thanks contribution