Previously we have capitalized input strings at different levels in
our code: in the user-facing API methods and in the asset finder.
This commit moves input string capitalization exclusively to the API
method to which the string was supplied. Specifically, the string is
capitalized by a preprocess API method decorator. The preprocess
decorator passes the input string to the newly defined
ensure_upper_case() method, which returns a TypeError if the argument
supplied is not a string.
ensure_upper_case() is defined in a new file, zipline/utils/input_validation.py.
The existing expect_types() method is also moved there.
Various tests in tests/test_assets.py are modified to account for the
fact that the asset finder method lookup_symol() no longer capitalizes
its supplied argument.
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