fix(request.header): core.request.header return string instead of table

[ronething]

Signed-off-by: ashing axingfly@gmail.com

Description

Fixes #11126

In order to prevent the caller from judging the data type by itself.

notice: After core.request.header function changed, we need to get original header value by using core.request.headers in file real-ip.lua and ua-restriction.lua

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[ronething]

@moonming please take a look at this PR when you have time, thanks.