odbc_result::as_double(date) should use UTC timezone

[shrektan]

Closes #293

It's because the user timezone info on date is meaningless. Converting to the user timezone then extracting the date part will leading to one-day leap if the timezone is UTC+n, like Asia/Shanghai, or "Europe/Berlin".

This PR will address this issue.

Example

(Before this PR, the result is "2018-12-31")

con <- DBI::dbConnect(
  odbc::odbc(),
  server = '#hide#', database = '#hide#', uid = '#hide#', pwd = '#hide#',
  encoding = 'GB2312', driver = '{SQL Server Native Client 11.0}', port = 1433,
  timezone = 'Asia/Shanghai'
)
out <- DBI::dbGetQuery(con, "select cast('2019-01-01' as date) as aaa")
str(out)
#> 'data.frame':    1 obs. of  1 variable:
#>  $ aaa: Date, format: "2019-01-01"

^{Created on 2019-08-26 by the reprex package (v0.2.1)}

[jimhester]

Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber), then I can merge this.

[shrektan]

I've added one item to NEWS. Thanks.

[jimhester]

Thanks!