v0.19.0

• Refactor the inner implementation.
• Added:
  • Add the BaseURL, Do methods for Client.
  • Add the Doer interface instead of *http.Client.
• Removed:
  • Remove the header constants.
  • Remove the ToError method from the Response type.
  • Remove the XxxJSON and XxxJSONContent functions, and use XxxContext instead.
  • Remove the unused Decoder type, and the CloseBody, GetContentType, NewRequestWithContext functions.
• Changed:
  • The Hook interface returns a new error type argument.
  • Remove the first argument code from the NewError function signature.