Add base64 chars sanitize function

[matdodgson]

In our use case we use our Nexus as a nodejs.org proxy with basic auth. The authorization header option should support the full base64 character set. I updated nvm_sanitize_auth_header to support this.

I also added a unit test.

I modified security_wget_injection as it was incompatible with supporting base64 encoding in the authorization header due to the "/tmp" assertion. I changed that test to check for dangerous shell metacharacters instead.