return correct content-types for swagger-ui files

[bihe]

I was using the "Secure middleware" from echo together with echo-swagger. The result was a blank screen.
The option X-Content-Type-Options: nosniff complained, that the *.js files did not have the correct Content-Type. Which is true as seen in the screenshot:

they were delivered using Content-Type: text/plain; charset=utf-8

I poked around and found that the gin-swagger implementation takes care of this (https://github.com/swaggo/gin-swagger/blob/master/swagger.go)

My PR just copies the logic of gin-swagger resulting in correct Content-Types which work with "nosniff"

[ubogdan]

closing: solved in #57