Refactored smells out of document/id.rb

Smells reported by reek:

Before:

19: NoBrainer::Document::Id#== performs a nil-check. (NilCheck)
38: NoBrainer::Document::Id#self.generate has approx 9 statements
    (TooManyStatements)
52: NoBrainer::Document::Id#self.generate has the variable name 'e'
    (UncommunicativeVariableName)
52: NoBrainer::Document::Id#self.generate has the variable name 'v'
    (UncommunicativeVariableName)

After:

19: NoBrainer::Document::Id#== performs a nil-check. (NilCheck)

Note:

With the behavior specified in equality_spec, this nil check is
required.