Repair IEX Market Data filtering, Stock endpoint tests

[addisonlynch]

• Repairs IEX Market Data Filtering (fixes IEX Market Data functions not filtering when symbol passed #46)
• Repairs Stocks endpoint tests (fixes Expected close test fails #45)

[codecov]

Codecov Report

Merging #50 into master will decrease coverage by 0.14%.
The diff coverage is 98.43%.

@@            Coverage Diff             @@
##           master      #50      +/-   ##
==========================================
- Coverage   96.33%   96.18%   -0.15%     
==========================================
  Files          13       13              
  Lines        1475     1496      +21     
==========================================
+ Hits         1421     1439      +18     
- Misses         54       57       +3

Impacted Files	Coverage Δ
tests/test_stock.py	100% <100%> (ø)	⬆️
tests/test_market.py	95.58% <100%> (-2.75%)	⬇️
iexfinance/market.py	94.02% <94.73%> (-0.42%)	⬇️

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