Explode string into ay

If the expected parameter is an array of bytes, and a string is
recieved, then explode the string into an array of bytes.
More saying error in case an array was expected
Test passing a string for a byte array (#40).

It fails on Ruby 1.9 because String#bytes is an Enumerator, not an
Array.
docs: small refactoring
docs: passing a string for a byte array (#40).
for DBus Arrays, Ruby Enumerables are enough
simplified condition

Use Type#child
test failing cases
Merge branch 'allow-string-for-byte-array'; fixes #40.