Fix error if timezone with name is used in parser

[jonasrutishauser]

What this PR does / why we need it:
Fixes a TypeError: nil can't be coerced into Integer when a timezone name is used in a parser configuration.

[repeatedly]

Hmm... Could you show the actual example?
timezone Europe/Zurich has this problem?

[jonasrutishauser]

Every zone name has this problem (see https://makandracards.com/makandra/67330-why-you-can-t-use-timezone-codes-like-pst-or-bst-for-time-objects).
With daylight saving time it is needed to use a zone name and the validation on line 155 accepts it whilst the parsing implementation on line 206 is not able to handle it.
And yes it is this config:

[...]
<parse>
  [...]
  timezone Europe/Zurich
</parse>
[...]

[repeatedly]

Thanks. I will check it.

[repeatedly]

Your patch has performance penalty and it is not good for existing users.
So need to rewrite the patch like below:

when timezone
  offset = Fluent::Timezone.utc_offset(timezone)
  if offset.respond_to?(:call)
    ->(t) { Time.now.localtime.utc_offset + offset.call(t) }
  else
    ->(t) { Time.now.localtime.utc_offset + offset }
  end

when strptime
  if offset_diff.respond_to?(:call)
    ->(v) { t = strptime.exec(v); Fluent::EventTime.new(t.to_i + offset_diff.call(t), t.nsec) }
  else
    ->(v) { t = strptime.exec(v); Fluent::EventTime.new(t.to_i + offset_diff, t.nsec) }
  end
# same as format

[repeatedly]

@ganmacs Could you also check this patch? I want to release v1.5.1 with this patch.

[repeatedly]

Thanks!