After we upgraded to version 1.5.1 we started to see issue with escaped values in header. Looks like it started to happen from version 1.5.0 because of this commit 78296c8.

Specifically this line (fix was for query, but it also called for header):

func styleStruct(style string, explode bool, paramName string, value interface{}) (string, error) {
   ...
   return prefix + url.QueryEscape(strVal), nil
}