Fixed an incompatibility with Ruby 1.9.
Fixed an incompatibility with Ruby 1.9.

Ruby 1.8 strings are Enumerable, but there is no String#lines method. In
Ruby 1.9, the situation is reversed.  To work around this disparity, the
RailsEnvironment#externals method now explicitly checks whether a String
responds_to? :lines.

[#2130 state:committed]

Signed-off-by: Jeremy Kemper <b3f594e10a9edcf5413cf1190121d45078c62290@bitsweat.net>

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