fix tests for nil TypeErrors for ruby 2.0=<

The text for nil to string conversions has gone from:

$ ruby -e '"" + nil rescue puts $!'
can't convert nil into String

to:

$ ruby -e '"" + nil rescue puts $!'
no implicit conversion of nil into String
Merge pull request #69 from geoffwa/bugfix/nil_typeerror_2.0

fix tests for nil TypeErrors for ruby 2.0=<