[BIRT-859] Fix bugs when write data out.

Two issue there:
1. FolderArchiveEntry didn't get flushed, when flushbuffer() is called.
2. Buffer mechnism error in RAStreamBufferMgr, leading to flush header
buffer every time, even in reader stream!

Signed-off-by: xinzhao-acutate <xzhao@actuate.com>

core/org.eclipse.birt.core/src/org/eclipse/birt/core/archive/FolderArchiveEntry.java

@@ -43,6 +43,7 @@ public void write( long pos, byte[] b, int off, int len )
 	{
 		out.seek( pos );
 		out.write( b, off, len );
+		out.flush( );
 	}
 
 	public void close( ) throws IOException

core/org.eclipse.birt.core/src/org/eclipse/birt/core/archive/RAStreamBufferMgr.java

@@ -47,15 +47,15 @@ public RAStreamBufferMgr( RandomAccessFile randomFile ) throws IOException
 		this.randomFile = randomFile;
 		this.length = randomFile.length( );
 		this.totalBuffer = 0;
-		this.currentBuffer = getBuffer( 0 );
+		this.currentBuffer = null;
 	}
 
 	/*
 	 * The file pointer in the underlying file if no buffer is used.
 	 */
 	public long getFilePointer( )
 	{
-		return currentBuffer.getOffset() + currentBuffer.getBufCur();
+		return currentBuffer == null ? 0 : currentBuffer.getOffset() + currentBuffer.getBufCur();
 	}
 
 	/*
@@ -65,6 +65,9 @@ public long getFilePointer( )
 	 */
 	public void write (byte b[], int off, int len ) throws IOException
 	{
+		if ( currentBuffer == null ) {
+			currentBuffer = getBuffer( 0 );
+		}
 		while ( len > 0 )
 		{
 			int ret = currentBuffer.write( b, off, len );
@@ -88,7 +91,7 @@ public void seek( long localPos ) throws IOException
 	{
 		long offset = ( localPos / IOUtil.RA_STREAM_BUFFER_LENGTH ) 
 							* IOUtil.RA_STREAM_BUFFER_LENGTH;
-		if ( currentBuffer.getOffset() != offset )
+		if ( currentBuffer == null || currentBuffer.getOffset() != offset )
 			currentBuffer = getBuffer( offset );
 		currentBuffer.setBufCur( (int)(localPos - offset) );
 		if ( localPos > length )