get_instruments() inserts commas for non-list symbol parameter

[kartiksubbarao]

Description of Bug
get_instruments unconditionally calls ','.join(symbols) even if symbols is not a list. This breaks the functionality and e.g. prevents symbol-regex queries from working:

schwab-py/schwab/client/base.py

Line 1105 in c20c42f

'symbol': ','.join(symbols),

Code to Reproduce
api.get_instruments('[A-M][^ ]*', api.Instrument.Projection.SYMBOL_REGEX)

Expected Behavior
All non-option symbols starting with A through M

Actual Behavior
No results

A simple fix is to change this line to:
'symbol': ','.join(symbols) if isinstance(symbols, list) else symbols,

[kartiksubbarao]

@alexgolec I didn't quite understand your response to the pull request, can you please elaborate? I was trying to provide something that supported the intent of your existing code (convert an array of symbols into a comma-separated string), while enabling the symbols parameter to be passed as-is for calls like symbol-regex.