Fix request payloads, date validation, and Excel dependency

[dharunashokkumar]

• Send valid JSON bodies in get_bhav_copy, get_option_chain and get_most_active_puts_calls: they were sending str(dict) with single quotes, which is not valid JSON and only worked because the ASP.NET deserializer is lenient. Now use json.dumps like every other endpoint.
• Fix validate_date_param swallowing its own errors: the range checks raised inside the same try block that caught them, so users always saw a misleading 'not valid value' message instead of the real reason (e.g. range > 365 days). Also allow single-day ranges (start == end) and remove the stray print.
• Remove leftover debug print(url) in get_trading_statistics.
• Replace xlrd with openpyxl: xlrd >= 2.0 only reads legacy .xls files, but every Excel endpoint here fetches .xlsx, so those functions failed on a clean install. Also add openpyxl to install_requires so pip users get a working Excel reader.
• Add regression tests for payload format and date validation.