Issue fix for spdlog #595. Conversion warning.

See: #595

On line 85 in file sinks/wincolor_sink.h:
back_color &= ~(FOREGROUND_RED | FOREGROUND_GREEN | FOREGROUND_BLUE |
FOREGROUND_INTENSITY);

'back_color' is of type 'WORD' (unsigned short) whereas a bitwise
complement/NOT returns an int. This results in a conversion warning with
-Wconversion enabled.

85:20: warning: conversion to 'WORD {aka short unsigned int}' from 'int'
may alter its value [-Wconversion] back_color &= ~(FOREGROUND_RED |
FOREGROUND_GREEN | FOREGROUND_BLUE | FOREGROUND_INTENSITY);

Possible solution:
We know that the result of ~(FOREGROUND_RED | FOREGROUND_GREEN |
FOREGROUND_BLUE | FOREGROUND_INTENSITY) is always within the limits of
an unsigned short so a simple cast should suffice (correct me if I'm
wrong):

back_color &= static_cast<unsigned short>(~(FOREGROUND_RED |
FOREGROUND_GREEN | FOREGROUND_BLUE | FOREGROUND_INTENSITY));

include/spdlog/sinks/wincolor_sink.h

@@ -82,7 +82,7 @@ class wincolor_sink: public  base_sink<Mutex>
         GetConsoleScreenBufferInfo(out_handle_, &orig_buffer_info);
         WORD back_color = orig_buffer_info.wAttributes;
         // retrieve the current background color
-        back_color &= ~(FOREGROUND_RED | FOREGROUND_GREEN | FOREGROUND_BLUE | FOREGROUND_INTENSITY);
+        back_color &= static_cast<WORD>(~(FOREGROUND_RED | FOREGROUND_GREEN | FOREGROUND_BLUE | FOREGROUND_INTENSITY));
         // keep the background color unchanged
         SetConsoleTextAttribute(out_handle_, attribs | back_color);
         return  orig_buffer_info.wAttributes; //return orig attribs