title: Get the mean of specific Landsat Band (i.e. B2, B3, B4) of each of the images from a ImageCollection
body: <p>Being more specific, I'm workin with an Image Collection of Landsat Surface Reflectance Tier 1 and I want to calculate the mean of the value of the Band 2 (blue) of the pixels for every single image from the collection. And get an CSV of the mean with its respective date (sorry for the bad english). After I filtered with geometry, date and cloud cover I tried this function that saw from another post (don't remember where I saw it)</p>
  
  <pre><code>def prom(image):
    dict = image.reduceRegion(ee.Reducer.mean(), geometry)
    return image.set(dict)
  mean = collection8.map(prom)
  print mean.aggregate_array('B2').getInfo()
  </code></pre>
  
  <p>The problem is that I can get the mean of other properties from the images like 'EARTH_SUN_DISTANCE', but when I try 'B2' I don't get any results. How can get that info?</p>