Fixes for type corruption in case statements

Author: zachmu

sql/types/conversion.go

@@ -642,6 +642,10 @@ func GeneralizeTypes(a, b sql.Type) sql.Type {
 	if b == Null {
 		return a
 	}
+	
+	if a == b {
+		return a
+	}
 
 	if svt, ok := a.(sql.SystemVariableType); ok {
 		a = svt.UnderlyingType()
@@ -716,6 +720,11 @@ func GeneralizeTypes(a, b sql.Type) sql.Type {
 	if IsNumber(a) && IsNumber(b) {
 		return generalizeNumberTypes(a, b)
 	}
+	
+	if IsText(a) || IsText(b) {
+		return a
+	}
+	
 	// TODO: decide if we want to make this VarChar to match MySQL, match VarChar length to max of two types
 	return LongText
 }