This closes #1096, memory usage optimization and another 4 changes

- Unzip shared string table to system temporary file when large inner XML, reduce memory usage about 70%
- Remove unnecessary exported variable `XMLHeader`, we can using `encoding/xml` package's `xml.Header` instead of it
- Using constant instead of inline text for default XML path
- Rename exported option field `WorksheetUnzipMemLimit` to `UnzipXMLSizeLimit`
- Unit test and documentation updated

1 files changed, 1 insertions, 1 deletions

diff --git a/calcchain_test.go b/calcchain_test.go
index 4956f60..6144ed5 100644
--- a/calcchain_test.go
+++ b/calcchain_test.go
@@ -5,7 +5,7 @@ import "testing"
 func TestCalcChainReader(t *testing.T) {
 	f := NewFile()
 	f.CalcChain = nil
-	f.Pkg.Store("xl/calcChain.xml", MacintoshCyrillicCharset)
+	f.Pkg.Store(dafaultXMLPathCalcChain, MacintoshCyrillicCharset)
 	f.calcChainReader()
 }