Change periods of the day replacements to be case-sensitive

[edmocosta]

The Go time parsing requires the period of the day value and the layout element to be in the same letter case.
For example, If pm in lower case is used as template, it expects the value to be also in lower case (am, pm), otherwise an error will be returned by the time.Parse functions.

This PR changes lunes to replace the day periods elements respecting the define layout case, for example:

layout: January 02 Monday, 2006, 03:04:05 pm
value: Febrero 25 jueves, 1993, 02:03:04 a.m.
lang: es-ES

Before this change, it would translate the value to February 25 Thursday, 1993, 02:03:04 AM (upper case AM), which would fail to parse using the time.Parse:

val, _ := lunes.Translate("January 02 Monday, 2006, 03:04:05 pm", "Febrero 25 jueves, 1993, 02:03:04 a.m.", "es-ES")

// it returns an error, as the "AM" value cannot be parsed as a "pm" layout element. 
time.Parse("January 02 Monday, 2006, 03:04:05 pm", val)

With this PR changes, the case is maintained, and the same Translate function call will return February 25 Thursday, 1993, 02:03:04 am (lower case am), making the time.Parse function work as exptected.

--

Note: The locale translations lookup for periods of the day are still being compared using case-insensitive, it didn't change. the only thing that was changed is the layout element replacement (AM, PM), that are now replaced using the same layout letter case.

[jsvd]

LGTM