Added a "Symbols" parameters to ListPricesService

[blarsy]

#380
Added the "Symbols" parameters as specified in https://github.com/binance/binance-spot-api-docs/blob/master/rest-api.md#symbol-price-ticker
Corresponding unit test "TestListPricesForMultipleSymbols" was added, too.

[adshao]

#380 Added the "Symbols" parameters as specified in https://github.com/binance/binance-spot-api-docs/blob/master/rest-api.md#symbol-price-ticker Corresponding unit test "TestListPricesForMultipleSymbols" was added, too.

Please check the review comments.

[blarsy]

Should be good to go.

[codecov]

Codecov Report

Merging #388 (314d175) into master (815670a) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
- Coverage   70.28%   70.11%   -0.18%     
==========================================
  Files          68       68              
  Lines        6391     6544     +153     
==========================================
+ Hits         4492     4588      +96     
- Misses       1434     1475      +41     
- Partials      465      481      +16     

Impacted Files	Coverage Δ
v2/ticker_service.go	78.94% <100.00%> (+1.80%)	⬆️
v2/liquidity_pool_service.go	61.73% <0.00%> (-2.76%)	⬇️
v2/client.go	80.44% <0.00%> (+0.51%)	⬆️

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