Fix WebServerSecure streamFile() (#4545)

* Fix WebServerSecure streamFile()

ESP8266WebServerSecure's streamFile was using the base class' method
which did not use SSL encrypt before transmitting, leading to failure.

Add a new template method and required support for
WiFiClientSecure::write(Stream&) (using a local temp buffer since the
SSL libs do not grok Arduino Streams at all).

Fixes #4544

* Match ClientContext buffer and yield() behavior

ClientContext sends out 256 bytes at a time and gives a yield after
each chunk to ensure the WDT doesn't fire.  Mimic that behavior in
WiFiClientSecure::write(Stream&).

libraries/ESP8266WiFi/src/WiFiClientSecure.h

@@ -43,6 +43,7 @@ public:
   uint8_t connected() override;
   size_t write(const uint8_t *buf, size_t size) override;
   size_t write_P(PGM_P buf, size_t size) override;
+  size_t write(Stream& stream); // Note this is not virtual
   int read(uint8_t *buf, size_t size) override;
   int available() override;
   int read() override;