Adds UTF-8 encoding when converting bytes to string

[rembjo0]

XML validation fails when the JVM does not use UTF-8 as the default
encoding. Added UTF-8 encoding in byte to string conversions in:

• SamlResponse - after base64decode SAMLResponse
• Util.convertDocumentToString()

[coveralls]

Coverage decreased (-0.08%) to 95.947% when pulling cb24c1b on rembjo0:topic-encoding into 26def30 on onelogin:master.

[pitbulk]

Thanks for solving that.

[rembjo0]

Thanks for the quick response and accepting.
There are two more pull requests comming with the last details we need to get an ADFS 2.0 integration up and running using a plain vanilla OneLogin build.