Environment variable name bug fix and documentation clarification

[fumoboy007]

Commit 1: support environment variable names that contain commas

From The Open Group specification:

These strings have the form name=value; names shall not contain the character ’='. For values to be portable across systems conforming to POSIX.1-2017, the value shall be composed of characters from the portable character set (except NUL and as indicated below).

TL;DR Commas are valid.

Commit 2: make it clearer that the name generation uses UPPER_SNAKE_CASE

The documentation (and the code/tests) misled me into thinking that the name generation used Sentence_Snake_Case because it includes examples such as HostName→Host_Name and MyURL→My_URL. Even though the next bullet point says the name will be transformed to upper case, I think it is clearer to communicate that in the examples themselves.

[qiangxue]

Great job! Thank you for your contribution!

[fumoboy007]

Thanks for merging and thanks for the great library. 😊