better performance of creating web mvc tags

[mkrichevsky]

Moving some tags to constants instead of creating every time should be obvious.
Creation of response status is slightly faster, as we're not casting anymore. More details in benchmark and results

[jkschneider]

@mkrichevsky Could you do me a favor and rebase this against 1.0.x?

[mkrichevsky]

@jkschneider done :)

[jkschneider]

Interestingly, I think your benchmark demonstrates that there is no real difference between casting the int and Integer.toString(..), but the code is more succinct anyway. Also, that's how it was done in the Spring Boot 2 equivalent class :)