Allow symbol (i.e. quoted identifier) as safe SQL string

[kamipo]

pluck(:id) / order(:id) are very common use case, and passed symbol
(i.e. quoted identifier) is obviously safe argument, but
:id.to_s.split(/\s*,\s*/).all? { |part| permit.match?(part) } is
useless and a bit expensive operation for each such safe symbols (will
make extra 2 mutable strings, 1 array, 1 proc).

This avoids the expensive operation to such safe symbols, it makes
pluck(:id) / order(:id) itself about 9% faster.

https://gist.github.com/kamipo/11d428b57f3629a72ae89c6f21721326

Before (93e6407):

Warming up --------------------------------------
    users.pluck(:id)     1.217k i/100ms
users.order(:id).to_sql  1.848k i/100ms
Calculating -------------------------------------
    users.pluck(:id)     12.239k (± 8.2%) i/s -     60.850k in   5.013839s
users.order(:id).to_sql  19.111k (± 7.5%) i/s -     96.096k in   5.064450s

After (this change):

Warming up --------------------------------------
    users.pluck(:id)     1.293k i/100ms
users.order(:id).to_sql  2.036k i/100ms
Calculating -------------------------------------
    users.pluck(:id)     13.257k (± 6.9%) i/s -     65.943k in   5.002568s
users.order(:id).to_sql  20.957k (± 7.6%) i/s -    105.872k in   5.086102s