A SAML protocol message is form-encoded by applying the
base-64 encoding rules to the XML representation of the message and placing the result in a hidden form
control within a form as defined by [HTML401] Section 17. The HTML document MUST adhere to the
XHTML specification, [XHTML]. The base64-encoded value MAY be line-wrapped at a reasonable length
in accordance with common practice.
This is part of SAML specification. You can not and shall not wish to change to UTF-8 format.
Hope this answer your question.