Description of problem:
Right now the recipient of a needinfo can't update the bug or comment without resetting the needinfo. But sometimes (many times?) it would be useful to update the bug without resetting the flag (for example, to add a comment that the information are being collected, or to change the value of a field).
This would avoid a tiring and useless "set needinfo/clear needinfo/set needinfo/ etc" sequence.

I propose to let the recipient choose to reset or not the needinfo, by enabling the same checkbox that is visible for non-recipients, the one which says:
"I am providing the requested information for this bug (Will clear all needinfo requests). ". The only difference would be the default value (checked for the recipient, non-checked for non-recipients.