ShellPkg/set: Fix EfiShellSetEnv to use case sensitive compare

ShellPkg/set: Fix EfiShellSetEnv to use case sensitive compare

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=777

Per Shell spec, the environment variable has a case-sensitive name.
But today's implementation of EfiShellSetEnv() compares the
environment variable name case insensitively, which causes variable
like "CWD" cannot be set due to "cwd" is pre-defined variable.

The patch fixes this issue.

The EfiShellGetEnv() doesn't have such issue because it will
call into ShellFindEnvVarInList() which uses StrCmp().

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Jim Dailey <jim_dailey@dell.com>