Update base64 dependency to 0.21 plus minor refactoring

[mina86]

Update base64 dependency to the latest 0.21 version and while doing that refactor some of the code where base64 is used. The refactoring has a few benefits:

• base64 encoding is now done into an on-stack buffer avoiding String heap allocation,
• Inserting Content-MD5 is now in a helper function avoiding code duplication,
• match is used rather than if-else-if chain so that self.command() is now called only once and
• there are now fewer ‘to_string’ calls reducing allocations.

[durch]

@mina86 thanks for raising, bumped the dependencies as part of general hygine