[BUG] okhttp-gson generation on windows creates a uncompilable build.gradle

Created by: rmadisonhaynie

okhttp-gson generation on windows creates an uncompilable build.gradle

This issue is related to the 'sourceFolder' param and the fact that by default it is build using File.separator, so when substituted in build.gradle.mustache when building on Windows into a string the Windows file seperator '' is interpreted as an escape character and the gradle file will not compile.
This file https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/build.gradle.mustache

Here: `{{#sourceFolder}} sourceSets { main.java.srcDirs = ['{{sourceFolder}}'] }

{{/sourceFolder}}`

4.0.0-beta2

You can reproduce by running grade generateClient on Windows here https://github.com/criteo/criteo-marketing-sdk-generator

The generated build.gradle file in the java project won't compile because the source folder is 'src\main\java'