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/stream.go b/stream.go
index 65d6b72..4bd721e 100644
--- a/stream.go
+++ b/stream.go
@@ -112,7 +112,7 @@ func (f *File) NewStreamWriter(sheet string) (*StreamWriter, error) {
 	}
 	f.streams[sheetPath] = sw
 
-	_, _ = sw.rawData.WriteString(XMLHeader + `<worksheet` + templateNamespaceIDMap)
+	_, _ = sw.rawData.WriteString(xml.Header + `<worksheet` + templateNamespaceIDMap)
 	bulkAppendFields(&sw.rawData, sw.worksheet, 2, 5)
 	return sw, err
 }