typescript-fetch-api: Don't set the Content-Type when using multipart-formdata

[grantyb]

The template sets the Content-Type header in all cases, but it should not when using the fetch() API for multipart-formdata, as per this MDN documentation:

https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest_API/Using_FormData_Objects

The issue is on this line:

openapi-generator-plus-generators/packages/typescript-fetch-client/templates/api.hbs

Line 133 in 1bf0196

localVarHeaderParameter.set('Content-Type', '{{{mediaType}}}');