protoc-c: preserve case in C enum value names (Issue #129)

there is some confusion with regard to the use of lower case letters in
enum values. take the following message definition:

    message LowerCase {
      enum CaseEnum {
        UPPER = 1;
        lower = 2;
      }
      optional CaseEnum value = 1 [default = lower];
    }

this generates the following C enum:

    typedef enum _LowerCase__CaseEnum {
      LOWER_CASE__CASE_ENUM__UPPER = 1,
      LOWER_CASE__CASE_ENUM__lower = 2
        _PROTOBUF_C_FORCE_ENUM_TO_BE_INT_SIZE(LOWER_CASE__CASE_ENUM)
    } LowerCase__CaseEnum;

note that the case of the enum value 'lower' was preserved in the C
symbol name as 'LOWER_CASE__CASE_ENUM__lower', but that the _INIT macro
references the same enum value with the (non-existent) C symbol name
'LOWER_CASE__CASE_ENUM__LOWER':

    #define LOWER_CASE__INIT \
     { PROTOBUF_C_MESSAGE_INIT (&lower_case__descriptor) \
        , 0,LOWER_CASE__CASE_ENUM__LOWER }

additionally, the ProtobufCEnumValue array generated also refers to the
same enum value with the (non-existent) upper cased version:

    const ProtobufCEnumValue lower_case__case_enum__enum_values_by_number[2] =
    {
      { "UPPER", "LOWER_CASE__CASE_ENUM__UPPER", 1 },
      { "lower", "LOWER_CASE__CASE_ENUM__LOWER", 2 },
    };

we should preserve the existing behavior of copying the case from the
enum values in the message definition and fix up the places where the
(non-existent) upper case version is used, rather than changing the enum
definition itself to match the case used in the _INIT macro and
enum_values_by_number array, because it's possible that there might be
existing working code that uses enum values with lower case letters that
would be affected by such a change.

incidentally, google's C++ protobuf implementation preserves case in
enum values. protoc --cpp_out generates the following enum declaration
for the message descriptor above:

    enum LowerCase_CaseEnum {
      LowerCase_CaseEnum_UPPER = 1,
      LowerCase_CaseEnum_lower = 2
    };
t/: add test case for Issue #129 from Oleg Efimov