minor: fix bson document size messages
minor: style fixes, no parens needed
minor: unnecessary duplicate declaration, already defined at module level
minor: BSON should not have code that depends on Mongo
RUBY-531 support and respect maxMessageSizeBytes
minor:  use default module scope for bson constants

DEFAULT_MAX_BSON_SIZE
DEFAULT_MAX_MESSAGE_SIZE
Merge branch 'ruby-526'