net/http/httpspan: make user agent key compatible with Splunk
net/http/httpspan: make user agent key compatible with Splunk

Only regex "word" characters are valid for Splunk field names:

http://docs.splunk.com/Documentation/Splunk/6.2.0/Knowledge/Managefieldtransforms#Regular_expression_syntax_and_usage

While the hyphen in the "user-agent" field is still searchable
using an underscore, it's better not to force users to make that
distinction.

Closes chain/chainprv#450.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain