BUG: single group series should preserve group name
Merge pull request #7313 from cpcloud/fix-single-group-extract

BUG: single group series should preserve group name
BUG: do not check that val != val and not equal inf/-inf since by def val not eq val is nan
Merge pull request #7315 from cpcloud/fix-dropna-with-inf-7314

BUG: isnull doesn't properly check for inf when requested
BUG: union should not try to sort inplace because platform impls differ as to when sorting occurs for objects that cannot be compared