Passing an array to .values() gets serialized into a comma-delimited string

Passing the values as separate arguments works fine:

`insert('user', ['id', 'name']).values(33, 'Fred')`

but passing them as an array:

`insert('user', ['id', 'name']).values([33, 'Fred'])`

resulted in the arguments being .toString()ed into a single string:

`INSERT INTO user (id, name) VALUES ('33,Fred')`

instead of separate values:

`INSERT INTO user (id, name) VALUES (33, 'Fred')`

This is fixed in 81dffa7. /cc @schuttsm

(This bug was found as a result of @dmmalam's question in #11)

I also added 6 tests to the test suite.