support for tz argument in time_floor

[era127]

In the time_floor(x, 'days') function when unit = 'days' it does not seem possible to specify the timezone when the day starts. It defaults to the tz of x. Is that an argument you would consider adding?

It is possible to specify the time zone in some operations such as in bigquery TIMESTAMP_TRUNC sql and would be nice to use in regular r through dplyr translations. For example:

TIMESTAMP_TRUNC(TIMESTAMP '2008-12-25 15:30:00', DAY, 'UTC')
converts to "2008-12-25 00:00:00 UTC"

TIMESTAMP_TRUNC(TIMESTAMP '2008-12-25 15:30:00', DAY, 'America/Los_Angeles')
converts to 2008-12-25 08:00:00 UTC which is 2008-12-25 00:00:00 America/Los_Angeles