Allow for lower case [default] section

[BernhardLenz]

The aws cli uses a lower case [default] notation for the default section. ini should allow an overwrite of the name of the default section, e.g. through ini.DefaultSection = "default"

To Reproduce
Add section [default] to ini file with some random key/value pairs
Loop over each key

for _, fileSection := range file.Sections() {
	sectionName := fileSection.Name() //sectionName will be DEFAULT even though it is named default in the file
	for _, key := range fileSection.Keys() {//no keys available even though the [default] section has entries

Overall though the uppercase lowercase spellings in the ini file need to be preserved though, hence the file cannot be completely converted into lowercase.