Properly capitalize exported go types (#7636)

Previously if the type was "myEnum" it would be written as

```
type myEnum string

const (
  DEMO MyEnum = "DEMO"
)
```

which would fail because of the capitalization difference. This fixes that.

modules/swagger-codegen/src/main/resources/go/model.mustache

@@ -5,7 +5,7 @@ import ({{/imports}}{{#imports}}
 	"{{import}}"{{/imports}}{{#imports}}
 )
 {{/imports}}{{#model}}{{#isEnum}}{{#description}}// {{{classname}}} : {{{description}}}{{/description}}
-type {{{name}}} {{^format}}{{dataType}}{{/format}}{{#format}}{{{format}}}{{/format}}
+type {{{classname}}} {{^format}}{{dataType}}{{/format}}{{#format}}{{{format}}}{{/format}}
 
 // List of {{{name}}}
 const (