Base64 encoding for Pharo 7 (avoid Base64MimeConverter)

[astares]

In Pharo 7 the class Base64MimeConverter should be avoided and it is better to use:

base64Decoded
base64Encoded

on String.

So GRPharoPlatform>>base64Decode: aString should be changed from

base64Decode: aString
	^ (Base64MimeConverter mimeDecodeToChars: aString readStream) contents

to

base64Decode: aString
	^ aString base64Decoded

[jbrichau]

Addressed in c88ff7e

[astares]

Thanks!