ENH: Coerce user input with API method decorator

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.

zipline/algorithm.py

@@ -85,6 +85,7 @@ from zipline.utils.api_support import (
     require_not_initialized,
     ZiplineAPI,
 )
+from zipline.utils.input_validation import ensure_upper_case
 from zipline.utils.cache import CachedObject, Expired
 import zipline.utils.events
 from zipline.utils.events import (
@@ -95,6 +96,7 @@ from zipline.utils.events import (
 )
 from zipline.utils.factory import create_simulation_parameters
 from zipline.utils.math_utils import tolerant_equals
+from zipline.utils.preprocess import preprocess
 
 import zipline.protocol
 from zipline.protocol import Event
@@ -738,6 +740,7 @@ class TradingAlgorithm(object):
             self._recorded_vars[name] = value
 
     @api_method
+    @preprocess(symbol_str=ensure_upper_case)
     def symbol(self, symbol_str):
         """
         Default symbol lookup for any source that directly maps the
@@ -770,6 +773,7 @@ class TradingAlgorithm(object):
         return self.asset_finder.retrieve_asset(a_sid)
 
     @api_method
+    @preprocess(symbol=ensure_upper_case)
     def future_symbol(self, symbol):
         """ Lookup a futures contract with a given symbol.
 
@@ -792,6 +796,7 @@ class TradingAlgorithm(object):
         return self.asset_finder.lookup_future_symbol(symbol)
 
     @api_method
+    @preprocess(root_symbol=ensure_upper_case)
     def future_chain(self, root_symbol, as_of_date=None):
         """ Look up a future chain with the specified parameters.
 
@@ -823,7 +828,7 @@ class TradingAlgorithm(object):
         return FutureChain(
             asset_finder=self.asset_finder,
             get_datetime=self.get_datetime,
-            root_symbol=root_symbol.upper(),
+            root_symbol=root_symbol,
             as_of_date=as_of_date
         )