Constant utl_http.http_version_1_1 causes compiler issues on older Oracle versions

[daust]

The constant utl_http.http_version_1_1 is not available on Oracle 11. Although this is clearly an old version, it could still work.
See: #112

I will duplicate the constant in my code so that I don't rely on the Oracle package:

  -- The HTTP protocol versions that can be used in the function begin_request
  HTTP_VERSION_1_0  CONSTANT VARCHAR2(64) := 'HTTP/1.0'; -- HTTP 1.0
  HTTP_VERSION_1_1  CONSTANT VARCHAR2(64) := 'HTTP/1.1'; -- HTTP 1.1

[daust]

Replaced all occurrences of utl_http.http_version_1_1 with the same constant in the package xlib_http so that it will also work in Oracle 11.