pgconn: fix parseKeywordValueSettings rejecting trailing whitespace

[alliasgher]

Summary

Fixes #2284.

parseKeywordValueSettings rejected valid keyword/value connection strings that had trailing whitespace or multiple spaces between values.

Root cause

After consuming each value, the parser advanced by one byte (s[end+1:] for unquoted values, s[end+1:] after the closing quote for quoted values). This skipped a single whitespace character but left any additional leading spaces in s. On the next iteration, the loop found no = in " " and returned "invalid keyword/value".

The same issue occurred with quoted values followed by any trailing whitespace, because s[end+1:] only moved past the closing ' and left the spaces.

Fix

• Trim leading whitespace from s once before entering the loop so that an entirely-whitespace input exits cleanly.
• After consuming each value (unquoted and quoted), use strings.TrimLeft(…) instead of [end+1:] to consume all subsequent whitespace. This matches libpq's behavior and the PostgreSQL documentation, which describes the connection string syntax as allowing spaces between key-value pairs.

Tests added

TestParseConfigKVTrailingWhitespace covers:

• single / multiple trailing spaces
• trailing tab
• quoted value followed by trailing spaces
• two key-value pairs with trailing spaces

[jackc]

Thanks!