[Haskell] Converting underscores to `'Underscore` seems unnecessary

Description

swagger-codegen for Haskell converts underscores to 'Underscore, which seems unnecessary as _ is valid Haskell identifier syntax. As underscores seem to be quite commonly used this can severely negatively impact the readability of generated code / code that uses it.

Swagger-codegen version

2.3.1

Swagger declaration file content or url

Any swagger declaration with underscores in it.

Command line used for generation

swagger-codegen generate -i <any_declaration_with_underscores> -l haskell

Steps to reproduce

Run: swagger-codegen generate -i <any_declaration_with_underscores> -l haskell

Look at generated Types.hs file and observe the large amounts of 'Underscore.

Related issues/PRs

Could not find any.

Suggest a fix/enhancement

Add specialCharReplacements.remove("_"); to https://github.com/swagger-api/swagger-codegen/blob/master/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/HaskellServantCodegen.java#L51 method.

Inspect HaskellServantCodegen.java around line 51, then reproduce the issue with the provided generate command and a Swagger declaration containing underscores. Check the generated Types.hs output; done means valid underscores are no longer rendered as repeated 'Underscore text.